1. Field of the Invention
The invention concerns a winding support for an electrical coil, and an electrical coil with such a winding support, as well as a production method for an electrical coil.
2. Description of the Prior Art
Superconducting coils are used to generate strong, homogeneous magnetic fields, with the superconducting coils being operated in a sustained short circuit current mode. For example, homogeneous magnetic fields with magnetic flux densities between 0.5 T and 20 T are required for nuclear magnetic resonance spectroscopy (MR spectroscopy) and for magnetic resonance imaging. These magnets are typically charged via an external current circuit and are then separated from the external power source since, in the resulting sustained short circuit current mode, a nearly lossless current flow occurs within the superconducting coil. The resulting strong magnetic field is particularly stable over time since it is not affected by the noise contributions of an external current circuit.
In known winding techniques for such coils, one or more superconducting wires are wound on support bodies, and different wire segments are maintained in contact with one another by wire connections with optimally low electrical resistance or via superconducting connections. For classical low-temperature superconductors such as NbTi and Nb3Sn with transition temperatures below 23 K, technologies exist to establish superconducting contacts to link wire segments and to connect the windings with a superconducting sustained current switch. The superconducting sustained current switch is thereby part of the electrical circuit of the coil and is placed in a resistive conductive state by heating, in order to inject an external current. After deactivating the heating and cooling down to the operating temperature, this part of the coil is also superconducting again.
High-temperature superconductors, also called high-Tc superconductors (HTS), are superconducting materials with a transition temperature above 25 K, and above 77 K for some material classes, such as cuprate superconductors. For HTS, operating temperature can be achieved by cooling with cryogenic materials other than liquid helium. HTS materials are particularly attractive for the production of magnetic coils for MR spectroscopy and magnetic resonance imaging, because some materials have high upper critical magnetic fields of over 20 T. Due to the higher critical magnetic fields, the HTS materials are in principle better suited than the low-temperature superconductors for the generation of high magnetic fields (of over 10 T, for example).
One problem in the production of HTS magnetic coils is the absence of suitable technologies to produce superconducting HTS connections, in particular for second-generation HTS (known as 2G-HTS). The 2G-HTS wires are typically present in the form of flat twin-lead cables. If resistive contacts are introduced between the superconducting twin-lead cables, the losses in the coil can no longer be ignored and the generated magnetic field noticeably drops over a time period of a few hours or days (see IEEE Transactions on Applied Superconductivity”, Vol. 12, No. 1, March 2002, Pages 476 to 479 and “IEEE Transactions on Applied Superconductivity”, Vol. 18, No. 2, June 2008, Pages 953 to 956).
In DE 10 2010 042 598 A1, a superconducting MR magnet arrangement is described that has a superconducting twin-lead cable that is provided in the longitudinal direction with a slit between the two ends so that said superconducting twin-lead cable forms a closed loop surrounding the slit. In the magnet arrangement, the superconducting twin-lead cable is wound on at least one double coil made up of two sub-coils that are arranged rotated counter to one another so that they generate a predetermined magnetic field curve in a measurement volume. The winding disclosed in DE 10 2010 042 598 A1 can be designed as a freely supported coil body or as a coil winding on a winding support.
Known winding supports typically have the shape of hollow cylinders, for example with circular base areas (footprints), in which the coil winding is wound on the outer surface of the hollow cylinder with a predetermined winding tension. For applications in magnetic resonance systems, the inner space of the hollow cylinder remains free and forms an externally accessible volume for receiving an examination subject. In the coil arrangement disclosed in DE 10 2010 042 598 A1, the use of a conventional winding support is problematic because, in a conventional winding technique, one end of the twin-lead cable comes to lie on the winding support and is pressed firmly onto that winding support by the winding tension of the subsequent windings (turns). Such a mechanical fixing of the twin-lead cable end on the winding support prevents a mobility of the individual sub-coils counter to one another. Rotation of the coils relative to each other, however, is sometimes needed in order to generate a predetermined joint magnetic field curve, and such rotation is hindered or prevented by this conventional winding technique.